1. Field of the Invention
This invention relates to a method of feedback-controlling the air-fuel ratio of a mixture supplied to an internal combustion engine by the use of an exhaust gas ingredient concentration sensor having an output characteristic approximately proportional to the concentration of an ingredient in exhaust gases, and a method of detecting deterioration of the exhaust gas ingredient concentration sensor for use in the air-fuel ratio control.
2. Prior Art
Conventionally, there has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 62-203951 an air-fuel ratio control method of feedback-controlling the air-fuel ratio of a mixture supplied to an internal combustion engine (hereinafter referred to as "supply air-fuel ratio) to desired air-fuel ratios dependent upon operating conditions of the engine, by the use of an exhaust gas ingredient concentration sensor having an output characteristic approximately proportional to the concentration of a specific ingredient in exhaust gases emitted from the engine (the so-called proportional output type sensor), which comprises the steps of calculating an air-fuel ratio correction coefficient based upon an output from the exhaust gas ingredient concentration sensor and a desired air-fuel ratio, calculating as a learned value the difference of the value of the calculated air-fuel ratio correction coefficient and a predetermined reference value in each of a steady condition of the engine and a transient condition thereof, and controlling the supply air-fuel ratio by the use of the calculated learned value.
According to the above proposed method, it is possible to compensate for aging changes in the operating characteristics of fuel injection valves, an intake pipe pressure sensor, etc. by means of the above learned value. However, if the exhaust gas ingredient concentration sensor per se has become deteriorated, the output of the sensor no more correctly represents the actual concentration of the ingredient. Therefore, such deterioration cannot be compensated for by means of the learned value.